1. Field of the Invention
The present invention relates to a ladder-type filter used for such as a mobile phone and a wireless LAN terminal.
2. Background Art
FIG. 6 is a schematic plan view showing a configuration of a conventional ladder-type filter. As shown in FIG. 6, conventional ladder-type filter 1 has elastic-wave resonators 3, 4 on the series arms and elastic-wave resonators 5, 6 on the parallel arms, arranged in a ladder shape on piezoelectric substrate 2. In ladder-type filter 1, elastic-wave resonator 5 on the parallel arm suppresses spurious emission by the transverse mode around a resonance frequency by interdigital electrodes 5a and 5b connected to each other in series and electrode cross width D1 of interdigital electrode 5a made different from electrode cross width D2 of interdigital electrode 5b. 
As information on prior art documents related to the invention of the application, the patent literature (WO2009/060594) is known for example.
To secure enough capacitance of elastic-wave resonator 5 in such conventional ladder-type filter 1, the number of electrode finger pairs of interdigital electrodes 5a and 5b needs to be increased. Accordingly, interdigital electrodes 5a and 5b are extended in the direction of arranging electrode finger pairs, resulting in a larger ladder-type filter 1.
Particularly, to use filter 1 for a low frequency band such as Band 8 defined by UMTS (Universal Mobile Telecommunications System), the distance (pitch) between electrode fingers needs to be increased, resulting in extremely large ladder-type filter 1.